This invention relates to PEM fuel cells, and more particularly to electrical current collectors (e.g. bipolar plates) therefor that are made from polymer composite materials, and have low contact resistance with adjacent diffusion media.
Fuel cells have been proposed as a power source for electric vehicles and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called xe2x80x9cmembrane-electrode-assemblyxe2x80x9d (MEA) comprising a thin, solid polymer membrane-electrolyte having an anode on one face of the membrane-electrolyte and a cathode on the opposite face of the membrane-electrolyte. The anode and cathode typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the carbon particles, and proton conductive material intermingled with the catalytic and carbon particles. One such membrane-electrode-assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of the present invention. The membrane-electrode-assembly is sandwiched between a pair of electrically conductive current collectors for the anode and cathode, which current collectors typically contain a number of lands that define a plurality of channels or grooves for supplying the fuel cell""s gaseous reactants (i.e., H2 and O2/air) to the surfaces of the respective anode and cathode.
Multi-cell PEM fuel cells comprise a plurality of the MEAs stacked together in electrical series and separated one from the next by a gas-impermeable, electrically-conductive current collector known as a bipolar plate. Such multi-cell fuel cells are known as fuel cell stacks. The bipolar plate has two working faces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack, and electrically conducts current between the adjacent cells. Current collectors at the ends of the stack contact only the end cells and are known as end plates.
A highly porous (i.e. ca. 60%-80%), electrically-conductive material (e.g. cloth, screen, paper, foam, etc.) known as xe2x80x9cdiffusion mediaxe2x80x9d is interposed between the current collectors and the MEA and serves (1) to distribute gaseous reactant over the entire face of the electrode, between and under the lands of the current collector, and (2) collects current from the face of the electrode confronting a groove, and conveys it to the adjacent lands that define that groove. One known such diffusion media comprises a graphite paper having a porosity of about 70% by volume, an uncompressed thickness of about 0.17 mm, and is commercially available from the Toray Company under the name Toray 060.
In an H2xe2x80x94O2/air PEM fuel cell environment, the current collectors are in constant contact with highly acidic solutions (pH 3-5) containing Fxe2x88x92, SO4xe2x88x92xe2x88x92, SO3xe2x88x92, HSO4xe2x88x92, CO3xe2x88x92xe2x88x92, and HCO3xe2x88x92, etc. Moreover, the cathode operates in a highly oxidizing environment, being polarized to a maximum of about +1 V (vs. the normal hydrogen electrode) while being exposed to pressurized air. Finally, the anode is constantly exposed to hydrogen. Hence, the current collectors must be resistant to a hostile environment in the fuel cell. Accordingly, current collectors have heretofore been either (1) machined from pieces of graphite, (2) molded from polymer composite materials comprising about 70% to about 90% % by volume electrically-conductive filler (e.g. graphite particles or filaments) dispersed throughout a polymeric matrix (thermoplastic or thermoset), or (3) fabricated from metals coated with polymer composite materials containing about 30% to about 40% by volume conductive particles. In this later regard, see co pending U.S. patent application, Fronk et al Ser. No. 09/456,478, filed Dec. 7, 1999 which (1) is assigned to the assignee of this invention, (2) is incorporated herein by reference, and (3) discloses current collectors made from metal sheets coated with a corrosion-resistant, electrically-conductive layer comprising a plurality of electrically conductive, corrosion-proof (i.e. oxidation-resistant and-acid resistant) filler particles dispersed throughout a matrix of an acid-resistant, water insoluble, oxidation-resistant polymer that binds the particles together and to the surface of the metal sheet. Fronk et al-type composite coatings will preferably have a resistivity no greater than about 50 ohm-cm and a thickness between about 5 microns and 75 microns depending on the composition, resistivity and integrity of the coating. The thinner coatings are preferred to achieve lower IR drop through the fuel cell stack.
Lightweight metals such as aluminum and their alloys have also been proposed for use in making fuel cell current collectors. Unfortunately, such metals are susceptible to dissolution in the hostile PEM fuel cell environment. Accordingly, it has been proposed to coat lightweight metal current collectors with a layer of metal or metal compound, which is both electrically conductive and corrosion resistant to thereby protect the underlying metal. See for example, Li et al RE 37,284E, issued Jul. 17, 2001, which is assigned to the assignee of the present invention, and discloses a lightweight metal core, a stainless steel passivating layer atop the core, and a layer of titanium nitride (TiN) atop the stainless steel layer.
The present invention relates to PEM fuel cells using current collectors made either entirely, or at least in part, from polymer composite materials such as described above. A high contact resistance exists between the diffusion media and current collectors made from such composite materials owing to the fact that the diffusion media is porous (ca. 60%-80% porosity) and the surface of the composite material engaging the diffusion media contains less than 70% (typically about 30% to about 40%) by volume of conductive filler (with the remainder being non-conductive polymer matrix material). Hence, current flow between the diffusion media and the composite material occurs only at sites where a conductive strand of the diffusion media contacts the conductive filler in the composite material. No current flows at sites (1) where a diffusion media strand meets the polymer matrix material of the composite, or (2) where a composite""s conductive filler meets a pore in the diffusion media.
The present invention reduces the contact resistance between the diffusion media and composite-containing current collectors in PEM fuel cells. More specifically, the present invention is directed to a PEM fuel cell having at least one cell comprising (1) a pair of opposite polarity electrodes each having a first face exposed to a fuel cell reactant and a second face engaging a membrane-electrolyte interjacent said electrodes, (2) a porous, electrically-conductive diffusion media engaging the first face for distributing reactant over, and conducting electrical current from, the first face, and (3) a current collector (e.g. a bi-polar plate) engaging the diffusion media for conducting electrical current from the media. The current collector comprises a polymer composite that has a first electrical conductivity and comprises corrosion-proof electrically conductive filler dispersed throughout an oxidation-resistant and acid-resistant, water-insoluble polymeric matrix. The polymeric matrix may be either a thermoplastic or a thermoset material, and will preferably be selected from the group consisting of epoxies, polyamideimides, polyether-imides, polyphenols, fluro-elastomers, polyesters, phenoxy-phenolics, epoxide-phenolics, acrylics, and urethanes. The electrically conductive filler in the composite (1) are preferably selected from the group consisting of gold, platinum, graphite, conductive carbon, palladium, rhodium, ruthenium, and the rare earth metals, and (2) may take many physical forms (e.g. elongated filaments, or spheroidal, flake, fibrillose particles or aggregates of such particles Discrete fibrilose particles will preferably be oriented generally in the direction current flows through the current collector, such as described in copending U.S. patent application Blunk et al., U.S. Ser. No. 09/871,189 filed May 31, 2001, which is assigned to the assignee of the present invention and is incorporated herein by reference. The invention is particularly effective with current collectors made from composites having fillers comprising conductive filaments (e.g. graphite or carbon) extending through the thickness of the current collector (i.e. in the direction of current flow through the collector).
The invention comprehends an oxidation-resistant and acid-resistant surface layer that covers the composite component of the current collector and engages the diffusion media. The surface layer has a second electrical conductivity that is greater (hereafter hyperconductive) than the conductivity of the underlying polymer composite material, and may be a discrete layer adhering to the composite or an integral layer formed by embedding additional conductive particles in the surface of the composite, or smearing existing filler from the composite over the exterior surface of the composite. The hyperconductive surface preferably has a resistivity at least one order of magnitude less than the underlying composite, and most preferably, at least 100 times less than the underlying composite, and serves to reduce the contact resistance between the polymer composite material and the diffusion media by shunting electrical current passing through the media to the conductive particles in the polymer composite that reside at the interface between the surface layer and the composite. The invention is seen to be most beneficial at low (i.e.  less than 150 psi) stack compression pressures, but is also effective at high (i.e.  greater than 200 psi) stack compression pressures, especially with polymer composites having conductive particle loadings less than about 70% by volume.
According to one embodiment of the invention, the entire current collector is made (e.g. molded) from the polymer composite material and then coated with the hyperconductive surface layer. In another embodiment, the current collector comprises a metal substrate (e.g. a stamped metal sheet) that underlies a layer of polymer composite material, which, in turn, is coated, with the hyperconductive surface layer of the present invention. In still another embodiment, the metal substrate comprises a first acid-soluble metal (e.g. aluminum) underlying a second acid-insoluble, oxidizeable metal (e.g. titanium or stainless steel), a polymer layer atop the second metal, and the hyperconductive layer of the present invention atop the polymer composite coating.
According to a preferred embodiment of the invention, the hyperconductive surface layer comprises a plurality of discrete, abutting, oxidation-resistant and acid-resistant, electrically-conductive particles (most preferably graphite) embedded in a surface of the composite so as to provide a higher concentration of conductive particles at the surface than throughout the remainder of the composite. Suitable alternative particles include gold, platinum, conductive carbon, palladium, rhodium, ruthenium, and the rare earth metals (i.e. the same particles as are used in the polymer composite). According to another embodiment, the hyperconductive surface layer comprises a continuous, oxidation-resistant, and acid-resistant, electrically-conductive film (e.g. metal, graphitic carbon, hyperconductive polymer composite, etc.) on the surface of the composite. The continuous film will preferably be vapor deposited, sprayed or electrolessly deposited onto the composite using conventional Physical Vapor Deposition (PVD), spraying or electroless (a.k.a. autocatalytic) deposition techniques well known in the art.
The present invention also comprehends a preferred process for making a PEM fuel cell current collector comprising the steps of forming the current collector at least in part from a polymer composite material comprising a plurality of electrically conductive first particles dispersed throughout an oxidation-resistant and acid-resistant, water-insoluble polymeric matrix, and adhering a sufficient quantity of second electrically conductive particles to a surface of the composite material to provide that surface with a conductivity significantly greater (i.e. orders of magnitude greater) than that of the underlying composite material. The second particles may be applied to the composite by spraying, brushing, sifting, fluidized bed immersion or the like, and may be imbedded in the surface by impingement or simply stuck to the surface while it is in a tacky state. According to a preferred method, the current collector is made by (1) coating an electrically conductive substrate (i.e. composite or metal) with a tacky layer of uncured or undried composite material comprising a plurality of electrically conductive first particles dispersed throughout an oxidation-resistant and acid-resistant, water-insoluble polymer, (2) depositing a plurality of electrically conductive second particles onto a surface of the tacky layer to increase the conductivity of the surface over the conductivity of the remainder of the composite material, and (3) curing/drying the uncured/undried coating material. Most preferably, the second particles will be sprayed (ala sand blasting) onto the composite material with sufficient pressure to embed the particles in the uncured/undried coating material. The coating is then cured/dried to anchor the particles in place. Following curing/drying, any unbonded particles are brushed or blown from the surface. Alternatively, the surface of the polymer composite may either be heated or wetted with a solvent prior to spraying to soften the surface and render it more receptive to embedding the second particles. According to still another alternative, the surface of a dried/cured polymer composite is gently abraded (e.g. with fine sand paper) to remove any polymer skin that may have formed over the conductive filler and to smear the abraded filler over the underlying surface so as to increase its conductivity over that of the bulk of the composite material underlying the surface.
The present invention reduces the contact resistance between a composite-containing current collector and the diffusion media, which, in turn, permits the making of PEM fuel cells which require less stack compression, are smaller, are more efficient and have lower heat loads. Reducing compression alone improves stack durability, permits the use of thinner side and end plates, improves gas flow under the lands of the flow field, and provides more uniform current distribution.